Earlier efforts to develop a vaccine for RS virus were directed primarily at genetic alteration of wild-type human RS virus. To date none of these efforts has proven successful. Recently an avian-adapted strain of human RS virus (HRSV), thought to be attenuated, was tested in chimpanzees; unfortunately, it was observed to be fully virulent. Previous investigations of bovine RS virus (BRSV) were extended, and six strains have now been evaluated in cotton rats. Several show promise in protecting animals from HRSV infection. Further evaluation in primates is contingent upon development of a technique for sampling the lower respiratory tract for RS virus by pulmonary lavage. Two species of monoclonal antibody have been tested in cotton rats for effectiveness in protecting against HRSV infection. As more antibody species are obtained, we plan to determine the role of each species in immunity.